Subtyping

This checks subtyping relationships when defining types.

Super Type's Typeidx

Super type's typeidx must be smaller than the sub type's typeidx:

(module
  (type (sub 1 (func)))
  (type (sub (func)))

  (type $z (sub $a (func)))
  (type $a (sub (func))))

Super Type Can't Be Final

Super type can't be implicitly or explicitly final:

(module
  (type $t (func)) ;; implicitly final
  (type $s (sub $t (func))))

(module
  (type $t (sub final (func))) ;; explicitly final
  (type $s (sub $t (func))))

Type Matching

The sub type must match the super type:

(module
  (type $a0 (sub (array i32)))
  (type $s0 (sub $a0 (struct))))